Toothpaste certified prepared without nitrogen (10)
I believe the answer is:
dentifrice
'toothpaste' is the definition.
(I've seen this in another clue)
'certified prepared without nitrogen' is the wordplay.
'prepared' is an anagram indicator.
'without' means one lot of letters goes inside another ('without' can be similar in meaning to 'outside').
'nitrogen' becomes 'N' (N is the chemical symbol for nitrogen).
'certified' is an anagram of 'detifrice'.
'detifrice' going around 'n' is 'DENTIFRICE'.
(Other definitions for dentifrice that I've seen before include "Tooth cleanser" , "cleaner" .)